In order to develop better Cu-Au selective OSP (organic solderability preservatives)

the procedure and principle of depositing of several selective OSP on printed circuit board(PCB) were studied for the first time. The mechanism of OSP reaction and selective depositing were investigated with UV thickness measure

SEM morphology survey and EDS element-composing analysis. The results indicate that the complexation reaction between substituted benzimidazole derivatives and copper (Ⅰ) compound needs some activation energy. The copper (Ⅱ) ion

ferric (Ⅲ) ion and benzimidazole all can lower this activation energy and have positive catalytic effect

while the zinc (Ⅱ) ion has no such capacity. Employing the catalytic property and no participating in OSP film formation of the ferric ion can endow OSP the selectivity between copper and gold. And no complexation between benzimidazole and gold atom can also be used to make this kind of selectivity as well. The selective OSP containing zinc ion plus pre-treatment process is obviously better than the selective OSP containing ferric ion. The OSP film formation procedure does not change the surface morphology of copper and gold.